1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of material, and, more particularly, from a fibrous web being processed into a paper product on a papermaking machine. Specifically, the present invention is a method for manufacturing resin-impregnated endless belt structures, having texturized outer surfaces and designed for use on a long nip press of the shoe type on a papermaking machine, and the belt structures manufactured in accordance with the method.
2. Description of the Prior Art
During the papermaking process, a fibrous web of cellulosic fibers is formed on a forming wire by depositing a fibrous slurry thereon in the forming section of a paper machine. A large amount of water is drained from the slurry in the forming section, after which the newly formed web is conducted to a press section. The press section includes a series of press nips, in which the fibrous web is subjected to compressive forces applied to remove water therefrom. The web finally is conducted to a drying section which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation to yield a paper product.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entering the dryer section. As the dryer drums are often heated from within by steam, costs associated with steam production can be substantial, especially when a large amount of water needs to be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long press nips of the shoe type has been found to be more advantageous than the use of nips formed by pairs of adjacent press rolls. This is because the longer the time a web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip which can be five to ten times longer in the machine direction, that is, in the direction in which the web travels through the paper machine, than one formed between two press rolls is formed. Since the long nip is five to ten times longer than that in a conventional two-roll press, the so-called dwell time of the fibrous web in the long nip is correspondingly longer under the same level of pressure per square inch in pressing force used in a two-roll press. The result of this new long nip technology has been a dramatic increase in dewatering of the fibrous web in the long nip when compared to conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 5,238,537. This belt is designed to protect the press fabric supporting, carrying and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious inner surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Belts of the variety shown in U.S. Pat. No. 5,238,537 are made by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness on at least the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press. It is specifically this coating which must have a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and fibrous web.
The base fabric of the belt shown in U.S. Pat. No. 5,238,537 may be woven from monofilament yarns in a single- or multi-layer weave, and is woven so as to be sufficiently open to allow the impregnating material to totally impregnate the weave. This eliminates the possibility of any voids forming in the final belt. Such voids may allow the lubrication used between the belt and shoe to pass through the belt and contaminate the press fabric or fabrics and fibrous web. The base fabric may be flat-woven, and subsequently seamed into endless form, or woven endless in tubular form.
When the impregnating material is cured to a solid condition, it is primarily bound to the base fabric by a mechanical interlock, wherein the cured impregnating material surrounds the yarns of the base fabric. In addition, there may be some chemical bonding or adhesion between the cured impregnating material and the material of the yarns of the base fabric.
Long nip press belts, such as that shown in U.S. Pat. No. 5,238,537, depending on the size requirements of the long nip presses on which they are installed, have lengths from roughly 13 to 35 feet (approximately 4 to 11 meters), measured longitudinally around their endless-loop forms, and widths from roughly 100 to 450 inches (approximately 250 to 1125 centimeters), measured transversely across those forms. It will be appreciated that the manufacture of such belts is complicated by the requirement that the base fabric be endless prior to its impregnation with a synthetic polymeric resin.
It is often desirable to provide the belt with a resin coating of some predetermined thickness on its outer surface as well as on its inner surface. By coating both sides of the belt, its woven base fabric will be closer to, if not coincident with, the neutral axis of bending of the belt. In such a circumstance, the internal stresses which arise when the belt is flexed on passing around a roll or the like on a paper machine will be less likely to cause the coating to delaminate from either side of the belt.
Moreover, when the outer surface of the belt has a resin coating of some predetermined thickness, it permits grooves, blind holes or other cavities to be formed on that surface without exposing any part of the woven base fabric. These features provide for the temporary storage of water pressed from the web in the press nip.
It will be appreciated that such grooves or blind holes are usually produced by graving or drilling in a separate manufacturing step following the curing of the resin coating.
The present invention provides a solution to this particular problem, that is, the necessity for a separate manufacturing step, which characterizes prior-art methods for manufacturing resin-impregnated endless belt structures having void volume in the form of grooves, blind holes and the like on their outer surfaces.
Accordingly, the object of the present invention is to provide a method for manufacturing a resin-impregnated endless belt structure having a texturized outer surface, which provides the outer surface of the belt structure with void volume for the temporary storage of water pressed from a fibrous web in a press nip. The texturized outer surface represents an improvement over the grooves, blind holes and the like of the prior art.
The present invention, therefore, is the method for manufacturing the resin-impregnated endless belt structure, and the resulting product, for use in the papermaking process where an endless belt, impermeable to water, oil and other fluids, and having a texturized outer surface with a plurality of indentations for the temporary storage of water pressed from a paper web, is desirable.
In a first embodiment of the method, an endless, permeable base structure having a length measured therearound in a longitudinal direction, and having an inside and an outside, is placed about a support structure. The support structure has a texturized surface, and is adapted to place the base structure under tension in a longitudinal direction. The xe2x80x9ctexturizedxe2x80x9d surface has a plurality of protruding elements which are separate and distinct from one another. The support structure may be either a cylindrical mandrel or a pair of carrying rolls.
While on the support structure, the base structure is totally impregnated with a polymeric resin material. The polymeric resin material is dispensed onto the outside of the base structure, and passes completely therethrough to contact the texturized surface of the support structure and to provide an impression of the texturized surface on the polymeric resin material on the inside of the base structure. The polymeric resin material also forms a layer on the outside of the base structure. The polymeric resin material is then cured and ground to provide it with a smooth, uniform surface, and the belt structure thus obtained is removed from the support structure. The belt structure is finally turned inside out to place the impression of the texturized surface of the support structure on the outside of the belt structure.
In a second embodiment of the method, an endless, permeable base structure having a length measured therearound in a longitudinal direction, and having an inside and an outside, is again placed about a support structure. In this embodiment, the support structure has a smooth, polished surface, and is again adapted to place the base structure under tension in a longitudinal direction. The support structure may again be either a cylindrical mandrel or a pair of carrying rolls.
While on the support structure, the base structure is again totally impregnated with a polymeric resin material. The polymeric resin material is dispensed onto the outside of the base structure, passes completely therethrough to contact the smooth, polished surface of the support structure, and forms a layer on the outside of the base structure. The polymeric resin material is then cured and ground to provide it with a smooth, uniform surface.
The base structure, now coated on one side, is then removed from the support structure; inverted (turned inside out) to place the coating on its inner surface; and reinstalled about the support structure. The polymeric resin material is again dispensed onto the outside of the base structure, forming a layer thereon. A medium having a texturized surface, such as a woven fabric belt, is then used to impress a corresponding texturized surface into the layer of the polymeric resin material on the outside of the base structure.
The polymeric resin material is then cured, and the belt structure thus obtained is removed from the support structure.
It follows that, when manufactured according to either method, the product belt comprises a base structure in the form of an endless loop having an outer side and an inner side. A polymeric resin material impregnates the base structure and renders the base structure impermeable to fluids, such as oil and water. The polymeric resin material also forms an inner layer on the inner side of the base structure. The inner layer has a smooth surface. On the outer side of the base structure, the polymeric resin material forms an outer layer which has a texturized surface impressed thereon either by a support structure having a texturized surface or by a belt having such a surface.